To minimise this fouling, various solutions can be implemented to limit metal deposits: preventive chemical cleaning or remedial mechanical cleaning (using hydraulic jets), replacement of material (brass by stainless steel or titanium alloy, which are more corrosion-resistant) in certain secondary system exchanger tube bundles, modification of the chemical products used for conditioning of the systems and an increase in the pH of the secondary system. Some of these operations must be authorised beforehand, because they imply discharges of some of the products used. Some chemical cleaning processes are still being tested to confirm that the chemical products utilised are harmless. Since the 1990s, EDF has been running a programme to replace the SGs with the most severely degraded tube bundles. The SG replacement campaign for 26 reactors with non-heat treated Inconel 600 alloy tube bundles has been completed. It is continuing with replacement of SGs on the 26 reactors in which the tube bundle is made of heat treated Inconel 600. 2.2.4 Assessment of Nuclear Pressure Equipment in operation The reactor pressure vessels ASN issues reports following the inspections made during each ten-yearly outage on the primary systems, the reactor pressure vessels in particular, which undergo numerous checks and a hydro-test during these outages. During the generic phase of the fourth periodic review of the 900 MWe reactors, EDF justified the in-service strength of the reactor vessels up to their fifth periodic safety review. The generic approach adopted by EDF consists in conservatively considering the mechanical properties of the vessel experiencing the worstcase irradiation embrittlement. EDF carried out fast fracture resistance studies taking account of the changes in the properties of the materials and is carrying out inspections to check there are no prejudicial defects in the steel during the ten-yearly outage of each reactor. This generic approach was submitted to the Advisory Committee for Nuclear Pressure Equipment (GPESPN) for its opinion on 20 November 2018, 15 October 2019 and 8 September 2020. The GPESPN examination concerned the defects analysed, the estimated irradiation ageing of the metal of the vessel, the thermomechanical analyses, the studies assessing the margins with respect to fast fracture, the classification of small primary break transients and justification of the level of residual stresses in the circumferential welds of the core shells. The studies carried out and the additional information provided at the request of GPESPN lead to a favourable conclusion regarding the ability of the reactor pressure vessels to function for a further ten years, subject to the result of the examinations performed on the occasion of the fourth ten-yearly outages of the reactors concerned. Cast elbow assemblies The dossier produced by EDF was examined by ASN with production of an opinion from the GPESPN on 23 May 2019. Following this analysis, ASN sent EDF requests for additional substantiation of the predicted behaviour of the aged material, identification of the flaws present in the cast elbow assemblies, analysis of the fast fracture margins and in-service monitoring of these components. EDF has provided substantiating documents for certain types of elbow assemblies and the replacement strategy envisaged for others. The situation of certain elbow assemblies it would be hard to replace has led to technical developments in the fields of non-destructive testing. Restoring the mechanical properties of these elbow assemblies via thermal regeneration has been studied in recent months, although no industrial process has yet been determined. THE PRINCIPLES OF THE REACTOR VESSELS IN-SERVICE STRENGTH DEMONSTRATION The regulations in force require in particular that the licensee: ■ identify the operating situations with an impact on the vessel; ■ take measures to understand the effect of ageing on the properties of the materials; ■ deploy resources to enable it to ensure sufficiently early detection of defects prejudicial to the integrity of the structure; ■ eliminate all cracks detected or, if this is impossible, provide appropriate specific justification for retaining such a type of defect as-is. IN-SERVICE MONITORING DEFICIENCIES CONCERNING THE SELF-BLOCKING DEVICES ON THE MAIN PRIMARY SYSTEM LINES The self-blocking devices are items placed between the civil engineering anchors and a line or component. These devices are designed to limit unwanted sudden movements. They thus allow slow movements of the secured components, offering no resistance to movement of thermal origin, for example when returning the reactor to service. However, they block accidental rapid movements such as those linked to an earthquake, opening of a valve, or a rupture. The self-blocking devices require regular checks, to ensure that they are in good condition and are not locked or incorrectly adjusted, which could mean they might not play their role when called on, or degrade the components in normal operation. During an inspection on the Saint‑Laurent-des-Eaux site, the ASN inspectors found that the settings of numerous self-blocking devices mounted on the primary system lines were outside the tolerance values, without this having been identified by EDF. ASN then decided to conduct a nationwide review to check whether this equipment was correctly monitored. This revealed that non-conformities were widespread, with major shortcomings in the monitoring of the self-blocking devices. ASN asked EDF to take the necessary measures to rapidly correct the deviations identified and implement an action plan to improve the monitoring of this equipment. EDF reported a generic significant event as a result of these inspections. In the event of seismic loading or a dynamic transient, these non‑conformities could lead to stresses on the components and, in the worst case, a break on the lines concerned. ASN Report on the state of nuclear safety and radiation protection in France in 2022 293 • 10 • The EDF Nuclear Power Plants 10 01 07 08 13 AP 04 06 12 14 03 09 05 11 02
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